As a method and apparatus for measuring a measurement target surface of an object to be measured by means of a non-contact laser displacement sensor, conventionally, a method and apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2000-298011 have been known.
This measurement apparatus has a configuration in which two laser displacement sensors are disposed so as to face each other vertically and a measurement target object and a reference mirror are disposed between the two laser displacement sensors. The measurement target object is disposed so that a measurement target surface thereof is positioned within a measurement area of the upper laser displacement sensor, and the reference mirror is disposed so that a smooth reference surface thereof is positioned within a measurement area of the lower laser displacement sensor.
In this measurement apparatus, while the measurement target object and the reference mirror are moved simultaneously in a predetermined measurement direction, displacement of the measurement target surface of the measurement target object and displacement of the reference surface of the reference mirror are each measured by the corresponding laser displacement sensors, and the difference between the measured measurement-target-surface displacement data and the measured reference-surface displacement data is calculated and the calculated displacement data is defined as actual shape data of the measurement target surface.
In measurement using a laser displacement sensor, it is necessary to relatively move the laser displacement sensor and a measurement target object appropriately by means of a moving mechanism. However, linear motion errors (for example, an error of motion straightness or an error caused by pitching, yawing or rolling) are inherent in the moving mechanism. Therefore, if merely displacement of the measurement target surface is measured using the laser displacement sensor, the liner motion errors are added to measured data, and therefore, it is not possible to perform accurate measurement.
Accordingly, in the above conventional measurement apparatus, the smooth reference surface and the measurement target surface are measured simultaneously and a value of the difference between these displacement data is defined as actual shape data of the measurement target surface. That is, because the reference-surface displacement data reflects linear motion errors of the moving mechanism, pure displacement data (that is, shape data) of the measurement target surface can be obtained by deducting the reference-surface displacement data reflecting the linear motion errors from the measurement-target-surface displacement data to which the linear motion errors are added.
Thus, according to the above conventional measurement method and apparatus, it is possible to obtain accurate shape data from which linear motion errors are removed.